tmf: Correctly print HTInterval in its toString()

tmf: Correctly print HTInterval in its toString()

The ASCII value of '[' would get added to the start time,
instead of doing the expected string concatenation.

Changed the whole thing to use a StringBuilder instead.

Change-Id: Iea7f2b409c7c177a4a0b35494294fca7258835a1
Reported-by: Patrick Tassé <patrick.tasse@gmail.com>
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/6985
Reviewed-by: Bernd Hufmann <bhufmann@gmail.com>
IP-Clean: Bernd Hufmann <bhufmann@gmail.com>
Tested-by: Bernd Hufmann <bhufmann@gmail.com>